How do I denoise an image sequence that has already been rendered?
You can try to connect the image (or image sequence) to the denoise node without using the Normal and Albedo sockets but the result won't be as good as it could be.

In general, Intel's OpenImageDenoise is the best option for now, see how to set it up properly: How to properly connect the Denoise node in Compositor?. Alternativly, you can try using other software, see: Denoising Animations in Cycles.

Next time, I suggest enable Denoising Data and set the file format to OpenEXR Multilayer which allows to store any additional passes like Denoising Normal, Denoising Albedo, etc. to the image file as well (without doing anything), see: Save all render passes to EXR image?